Recent interest in high-resolution tomography has stimulated interest in high-frequency short wavelength energy sources. Two general types of transducers have emerged, one employing piezoelectric principles and the other electric arc discharge principles. Regardless of type, these prior transducers are relatively expensive to manufacture, frequently application specific, and lack a repeatable seismic signature in all geologic settings.
Piezoelectric transducers transmitting a continuous signal in a frequency band of 300 to 2,500 Hz. at an acoustic power of 10 W have been deployed in a manner similar to that of vibrator style surface seismology. Cross-well seismic measurements in sedimentary rocks have been employed, particularly when a piezoelectric cylindrical bender bar transducer operating with gated sinusoidal pulses has been used.
Arc discharge transducers capable of generating pulse signals having dominant frequencies within the 1-2 kHz. range with sufficient energy to allow pulse signal detection at distances of 300 meters or more in relatively low quality rock are known. High-frequency arc discharge devices have principally been used for sub-bottom profiling, although borehole versions of marine sparking devices are known.
One problem with prior seismic sources has been the cost of the equipment, which may be excessive in the event the transducer cannot be retrieved from the borehole. In that event, the transducer is lost and all its costs wasted. Yet another problem is the fact that these seismic sources are generally application specific, so that each is custom engineered and therefore essentially not useable in some other borehole.
Those skilled in the art will appreciate that there is a need for a high-frequency borehole seismic source which is relatively inexpensive to manufacture so that loss in a borehole is tolerable, the source is not application specific so that a single unit may be used in any number of boreholes, and the source has excellent repeatability of seismic signature in a variety of geologic settings. The disclosed invention meets these criteria in a device which may be manufactured and assembled from commercially available components which are relatively inexpensive.